As is well known in the gas turbine engine art, it is typical to include variable stator vanes in certain stages of compression in the compressor section. In order to enhance engine performance, reliability and power output, the angle of the vanes are varied to a particular schedule during the operating envelope. Compressor efficiency is maximized by orienting the angle of attack of the engine working fluid before flowing to the compressor blades of the compressor rotor. This requires angular changes of each of the vanes in a stator row of vanes. In order to effectuate this change, a unison or synchronizing ring (sync ring) by way of linkages is attached to each of the vanes and an actuator(s), scheduled by the engine control, through a stator linkage system including a pumphandle and slider bracket to mechanically position the sync ring(s).
U.S. Pat. No. 4,755,104 granted to J. H. Castro and R. S. Thompson on Jul. 5, 1988 entitled "Stator Vane Linkage" and assigned to United Technologies Corporation, the assignee common to this patent application, describes a typical variable stator vane system of the type which is a concern in this invention. As noted in this patent, adjustment of the individual vane is carried out by a mount rotatable about a radially oriented axis linking each blade of an individual stage together by a plurality of corresponding vane arms extending perpendicular to each axis of rotation for each blade. Each arm further being joined at the end thereof to the sync ring encircling the generally cylindrical compressor case and causing equal radial rotation in each linked stator vane in response to relative circumferential displacement between the unison ring and the compressor case.
Problems, particularly in maintenance, replacement of and the wear on the stator vane system, have occurred resulting in misscheduling of the stator vanes. In other words, when the linkages, components or actuators, are reassembled under the current rigging procedure (the procedure for setting the vane angle relative to the linkage and actuator) mischeduling problems have occurred where the angle of the vanes is no longer correlated to the input signal of the actuator. This problem is also a result occasioned from the wear of certain component parts of the stator linkage system.
To appreciate the problem, it is best to understand the rigging procedure for the heretofore known stator vane system design. A typical system consists of an external bellcrank that is actuated by an externally mounted hydraulic actuator. Generally an actuator mounted on the wall of the fan duct or the compressor case is connected to an externally mounted bell crank that, in turn, rotates an internal bellcrank through a torque shaft configuration. The internal bellcrank is connected to a pumphhandle by a link which rotates about a pivot bolt, and a slider bracket mounted to the engine case establishes the plane of rotation. The pumphandle, in turn, is connected to a series of sync rings through an equal number of links. A single engine will typically employ two of these systems equally spaced around the compressor.
What has been described immediately above is conventional and well known technology.
The procedure for rigging this assembly is as follows: The internal bell crank is rotated until a rigging hole in the pumphandle is aligned to a rigging hole in the slider bracket. A pin is temporarily inserted into the holes to hold the pumphandle in place relative to the slider bracket. An adjustable stop screw mounted on the slider bracket is then adjusted to contact with the pumphandle and locked down with a jam nut. At this point the rig pin is then removed. This now represents the rigged (open) position of the pumphandle, snyc rings and vanes. This procedure is repeated on the other side of the compressor. In installations where the actuator is affixed to a fan duct, the fan duct can now be installed and the external bellcrank is inserted through the fan duct and secured to the internal bellcrank. In other installations the actuator and external bellcrank are connected directly to the compressor case. In either embodiment, the final rigging procedure is to then torque the external bellcrank until the pumphandle contacts the set screw. The clevis of the actuator is then turned until it aligns with the external bellcrank (with the actuator fully retracted) and then bolted in place. Ideally, this would allow the actuator and pumphandle to contact their stops simultaneously.
As mentioned herein above, this system has evidenced problems occasioned by using the wrong size rigging pin, over torquing the stop screw (thus yielding the pumphandle), not contacting the pumphandle with the stop screw, over-torquing the external bellcrank (also yields the pumphandle), and a series of other human error mistakes all of which result in mischelduled variable vanes. Since the position of the vanes affects the angle of attack of the working fluid medium, the operation of the compressor is adversely affected.
I have found that I can obviate the problems noted above and eliminate the complex rigging procedure alluded to in the paragraph immediately above as well. In accordance with my invention, a fixed stop is machined on the slider bracket to which the pumphandle will contact when its at its correct rigging position. This creates a fixed rigging reference point and the rigging holes are thusly, eliminated. Since the contact areas on the pumphandle and slider bracket can be machined to the same tolerance as the rigging holes, there will be no increase in vane misposition due to manufacturing and assembling tolerances. The vanes will be set to their correct positions when the hardware is bolted to the case, and no further internal rigging is required.
Another problem that is evidenced in the heretofore known variable stator vane actuating systems is that as a result of the misrigging the contact stresses occasioned by the adjustable stop screw contacting the pumphandle prior to the actuator hitting its stop, continuing force of the actuator results in an significant over yield of the pumphandle. For the reasons enumerated above, the misrigging causes the stops on the actuator and pumphandle to become out of sync. Ideally, the stops should hit simultaneously if the system is rigged correctly. The problem is even further acerbated in a turbofan installation where the actuator is mounted on the fan duct. In this type of installation the thermal growth differences between the fan duct and the actuator causes the pumphandle to contact the stop screw prior to the actuator hitting its stop which causes compressive yielding of the pumphandle. Obviously, the problem compounds every time the actuator is removed for service without sliding the duct to rerig the system. When the actuator is reinstalled, the external bellcrank is torqued to where the stop screw contacts the pumphandle which is now displaced as a result of the yielding and the actuator clevis is adjusted to fit on the external bellcrank. The system is now misrigged and the wear/yielding cycle starts again. The heretofore known systems typically place a crown configuration on the contact portion of the stop screw which causes very high contact stresses when the pumphandle is loaded against the stop screw, resulting in pumphandle yielding. Attempts to obviate this problem by increasing the contact area of the stop screw/pumphandle so as to lower contact stresses, have been unsuccessful to prevent the yield problem.
This invention obviates this yield problem by making the width of the machined fixed stop of this invention sufficiently large so that the contact stresses are well within acceptable limits for both the pumphandle and slider bracket.